Generally, a method for producing a silicon wafer comprises a slicing process wherein a single crystal ingot pulled by the Czochralski method or the like is sliced to provide a thin disk-like wafer, a chamfering process of chamfering the peripheral part of the wafer produced in the slicing process in order to prevent cracking and chipping, a lapping process of planarizing the wafer, an etching process of removing mechanical damages which remain on the wafer after being chamfered and lapped, a polishing process of making the surface of the wafer to be a mirror surface, and a cleaning process of cleaning the wafer to remove a polishing agent or foreign matter adhered on the wafer.
In the above-mentioned method for producing the silicon wafer, there were shown the main processes. There may be added the processes, such as a surface grinding process and a heat treatment process or the like. Moreover, the order of the processes may be changed, or the same processes may be repeated plural times in some cases.
Then, the wafer is subjected to inspection or the like, and is sent to a device manufacturing company (process), where an insulator layer or metal wiring is formed on the silicon wafer, to produce a device, such as memory.
In manufacture of a silicon wafer, there may sometimes arises waiting time between each of the processes before being supplied to the following process, because of capacity of the apparatus, trouble of the apparatus, or the like. In this case, it is necessary to store it by a method suitable for the condition of the silicon wafer. For example, there is a cleaning process as the following process after a polishing process, and the wafer is usually stored in water during the waiting time until it is transferred to the cleaning process. This is because, if the wafer is stored in the atmosphere, solidification of the slurry is caused as a result of dryness of a polishing slurry, which is difficult to be removed at the cleaning process as the following process.
Moreover, for example, in the case that the polishing process is conducted according to a wax mount method wherein the wafer is held on a holding plate with a wax and polished, there is a process wherein the wafer is peeled from the holding plate with pouring a shower water to the wafer after polishing. Moreover, showering of the wafer is conducted by pouring a shower water on the wafer, even if it is not conducted according to the wax mount method.
On the other hand, the need for improvement of the quality of a wafer has become still severer. For example, it has become necessary to pay attention fully to metal contamination and surface roughness on the surface of the wafer. For example, oxide dielectric breakdown voltage (GOI: Gate Oxide Integrity) and haze (HAZE: called micro roughness) or the like are measured as an item of wafer quality, and the wafer is determined whether it is good or not. Even if it is the wafer which is satisfactory in the present quality standard of the wafer, the yield may be lowered in a device manufacturing process or the like. There may be various reasons why yield is lowered even if the wafer satisfies a certain standard. It has turned out that it is influenced a lot especially by showering to the wafer and storage of the wafer in liquid after a polishing process, in a wafer manufacturing process. Since the surface of the wafer immediately after polishing is active, and thus the possibility that the influence of atmosphere, storage water or the like lead to degradation of wafer quality is large. Furthermore, since it is the last process of manufacture of the wafer, it is difficult to improve the quality of the wafer surface thereafter. On the other hand, it is indispensable to subject a silicon wafer to showering and storage in liquid in order to prevent solidification of a polishing slurry or the like. Therefore, it is necessary to pay attention fully to storing and showering immediately after polishing.
For this reason, each manufacturing process of a silicon wafer has been conventionally performed in a clean room where environmental condition such as air cleanliness, atmospheric pressure and temperature, was controlled and managed severely. In the clean room, the room temperature has been kept at 20 to 25° C., and storage water and shower water have also been controlled at a temperature of 20 to 25° C. However, there was a case where the quality of a wafer is degraded also under such control.
Although the cause of degradation of the wafer quality is not clear, it is considered to be slight roughened surface or slight metal contamination on the surface of the wafer caused by a storage water or a shower water, or other action on the surface of the wafer.
Therefore, it is required for a chemical liquid used for the storage (a storage water), or a shower water to have almost no etching effect. The chemical liquid used for cleaning or the like usually has an effect of etching a wafer surface slightly, and the surface condition of the wafer changes with time. Since storage time varies depending on the condition of the process when the wafer is stored in the storage water, there should be no influence by storage time in order to prevent variation and to maintain the same condition of the surface of the wafer.
Accordingly, there have been developed as a method for storing the wafer after polishing: a method of immersing it in hydrogen peroxide water to prevent contaminant on the surface of a wafer (Japanese Patent Application Laid-Open (Kokai) No. 7-263403), a method for storing it in a pure water which contains ozone in order to prevent deterioration of surface roughness (Japanese Patent Application Laid-Open (Kokai) No. 8-83783), a method of controlling metal (Cu) concentration in the storage water in order to prevent breakdown voltage failure (Japanese Patent Application Laid-Open (Kokai) No. 11-191543), a method of adding a chelating agent, in order to prevent breakdown voltage failure by metal contamination (Japanese Patent Application Laid-Open (Kokai) No. 11-243073), a method of using an electrolysis cathode water or an electrolysis anode water as a storage water in order to remove particles (Japanese Patent Application Laid-Open (Kokai) Nos. 2000-49127 and 2000-49128), and a method of adding surfactants or citric acid or the like.
By using the storage water as described above, it is possible to prevent metal contamination and to prevent deterioration of surface roughness, and to prevent degradation of the wafer quality such as an oxide dielectric breakdown voltage and haze to some extent. However, the wafer quality evaluated according to the following SC1 evaluation method may be varied in some cases, even if the above-mentioned storage water is used, and attention is paid to the cleanliness of shower water, and the environment in a clean room.
The SC1 evaluation method is a method of estimating a wafer quality by etching the surface of the wafer repeatedly (or for a long time) with a chemical liquid consisting of aqueous ammonia, hydrogen peroxide water, and water, actualizing the defects on the surface of the wafer, and then checking the number or the state of increase of LPD(s) (Light Point Defect) (called SC1-RT or SC1 long-time etching evaluation method or the like, hereinafter referred to as just “SC1 evaluation method”).
It is known that, when evaluating according to this method, there will be detected defects in a wafer, COP (Crystal Originated Particles) which is a crystal defect mainly generated while a silicon ingot is grown, or a mechanical damage formed during processing, and defects due to external contamination sources such as metal. However, it is thought that defects other than them which influences wafer quality are detected with high sensitivity.
Specific cause of degradation of the wafer quality by storage water or shower water is unclear. However the difference between good products and failure products can be confirmed by above SC1 evaluation method.